The broad band laser emission observed from organic dye lasers provides for a source of continuously tunable coherent radiation. Tuning of a dye laser was first accomplished in 1967 by Soffer and McFarland by replacing the back reflector of the optical resonant cavity with a rotatable diffraction grating. Since that time, other organic dye laser tuning techniques for producing narrow-band laser emission have been devised, most of which have been mechanical or electro-mechanical in design.
The disadvantages in using any tuning system that has moving parts, such as a rotatable diffraction grating or electro-mechanical devices, is the limited wavelength scan rate. The present invention is a tuning system having no moving parts. That is, tuning is accomplished by employing stationary components, which are a polarizer and a retroreflective, acousto-optic, electronically tunable filter (RETF). Laser beam diffraction is responsive to the radio frequency or frequencies (RF) applied to the RETF, and the RF "white noise" intentionally provided with a "notch" therein. The laser cavity maintains a high Q for only the wavelength corresponding to the notch. Thereby, we have notch scanning by laser wavelength.
Prior work has been done on electrically tunable filters by such as messieurs Taylor, Harris and Nich and Hansch, whose findings were reported in Applied Physics Letters, Volume 19, No. 8, of 15 Oct. 1971, under the title "Electronic Tuning of a Dye Laser Using the Acousto-Optic Filter." The work described therein contributes to the foundation of knowledge upon which the present invention rests. Tuning was achieved therein by applying an RF signal to an acousto-optic filter utilizing a collinear interaction between an ordinary optical wave, an extra-ordinary optical wave, and a travelling acoustical wave in a birefringent crystal. The optical frequency to which the laser is tuned corresponds to the single acoustic frequency applied to the crystal. Only the corresponding optical frequency undergoes a 90.degree. polarizing rotation to be provided as the output. All other optical frequencies are effectively blocked. That is, the RF frequency applied causes the corresponding optical frequency to undergo a 90.degree. polarization rotation, which polarized frequency is the one transmitted by the filter and provided as the tuned output of the laser. All other frequencies are not related and are therefore, blocked by the filter.